Electronic keyboard instrument



Dec. 27, 1949 T. K. RIGGEN 2,492,320

ELECTRONIC KEYBOARD INSTRUMENT Filed May 9, 1947 INVEN TOR.

WITNESS: B [ZZZ/Z. QZ ZL Y g Patented Dec. 27, 1949 ELECTRONIC KEYBOARD INSTRUMENT Theodore K. Riggen, Ehnira, N. Y., assignor to Bendix Aviation Corporation, a corporation of Delaware Application May 9, 1947, Serial No. 747,100

3 Claims. 1

The present invention relates to an electronic keyboard instrument and more particularly to an instrument having a keyboard similar to a piano in which the audible response to the actuation of a key is generated and controlled by electronic means.

It is an object of the present invention to provide a novel electronic keyboard instrument which is simple in construction and versatile in application and use.

It is another object to provide such a device which is particularly adapted for use in supervised practice and instruction.

It is another object to provide such a device which permits simultaneous operation of a plurality of instruments in a single classroom without interference while allowing monitoring, and individual or collective instruction.

It is another object to provide such a device which is so arranged that a single set of tone oscillation generators may be used in connection with any desired number of practice keyboards.

It is another object to provide such a device including a supersonic switching device which permits the use of a single amplifying sound system for all the notes of an instrument.

Further objects and advantages will be apparent from the following description taken in connection with the accompanying drawing which illustrates diagrammatically a preferred form of the invention.

In the drawing, a cathode-ray tube indicated generally by numeral l is provided with a cathode 2, a beam forming anode system 3, and a target surface 4 of conventional form. A plurality of conductive plates 5 are located on the surface of the target 4, preferably in the form of a circle. When a standard keyboard is to be formed, there will be 88 of these conductive plates 5.

Means are provided for causing the beam of the cathode ray 2 to momentarily impinge upon the conductive plates 5 in sequence so as to constitute a switching means capable of operating at a supersonic frequency. It will be understood that this may be readily accomplished by utilizing the deflector plates of the cathode ray tube in the well known manner to produce a circular motion of the beam, the deflector plates and sweep circuit systems not being illustrated for the sake of clarity.

A return circuit from each plate 5 back to the cathode 2 is provided comprising a lead 6, resistor I, lead 8, condenser 9 and resistor II in parallel, and ground connection 12; the circuit being completed through ground connection l3 and a resistor M which is common to all the plate return circuits.

A tone frequency oscillator indicated generally at numeral I5 is arranged to impress its output across the resistor 1 whereby its fixed frequency oscillation is superimposed on the plate current traversing said return or tone circuit.

Variations in plate current in the tone circuits produce corresponding signal voltages across the common resistor l4 and means are provided for amplifying such signal and operating a sound system therefrom.

As here shown in simplified form, this is accomplished by means of an amplifier tube l6 having a cathode ll, a grid l8 and a plate l9 which is connected by a lead 2| to a suitable sound producing device 22 such as a pair of earphones which are connected to the positive terminal of the amplifier plate battery 23.

The grid l8 of the amplifier tube I6 is connected through a filter choke '24 to the cathode 2 of the cathode-ray tube by a lead 25 so that the signal appearing across the common resistor I4 is applied to the grid l8 of the amplifier tube to modulate the output thereof and produce a correspondin audible signal in the earphones 22. The lead 25 is preferably connected to the ground at 26 through a filter condenser 30.

Means are provided for normally preventing the tone oscillator I5 from actuating the sound amplifier system. As here shown, this is accomplished by means of a resistor 21 in the ground lead of the cathode of the amplifier tube IS, the cathode I! being grounded at 28 and the negative terminal of the plate battery 23 being grounded at 29 to complete the plate circuit of the amplifier.

The value of the resistor 21 is such that the cathode I1 is biased positively with respect to the grid i8 sufliciently to render the amplifier insensitive to the signal appearing across the resistor l4 during the static or idle condition of the tone circuits. If deemed desirable a resistor 30 may be connected between the positive terminal of the battery 23 and the cathode I! of tube IS in order to maintain the desired bias on said cathode.

A key 3| is provided corresponding to each of the tone circuits, and means actuated thereby for rendering the sound amplifying system operative when it is connected to that particular tone circuit by the cathode-ray switching means. As here shown, this means comprises a source of direct voltage 82, the negative terminal of which is grounded at 33 and the positive terminal connected by a lead 34 to a spring mounted contact 35 which is normally engaged by the conductive portion 36 of the key 3|. Conductor 35 is connected by lead 31 to a storage or tank condenser 38 which is grounded at 39 so that the condenser is normally maintained charged with its positive plate connected to the key 3|. A contact 4| mounted on a spring 4!] is arranged in the path of movement of the key 3| so as to be engaged thereby after initial depression of the key. A third spring-mounted contact 42 is normally engaged with the spring of contact 4|, and is connected by a lead-43 to the lead 8 of the corresponding tone circuit.

A contact 44 which is grounded at 45 is arranged in the path of the spring 40 so as to ground the contact 4| when the key 3| is fully depressed. A resistor 46 is bridged across contacts 4|, 44 to form a bleeder connection therebetween- In operation, when the keys 3| are all in their normal or upper position, no sound is emitted by the earphones 22 since the bias caused by the resistor 27 prevents theoscillations of the tone generators from actuating the amplifier. If now a key 3| is depressed the corresponding storage or tank condenser, 38, is first disconnected from the power supply 32 and then connected through contact 4| andcontact. 42-to the supply or bucket condenser. 9 which is immediately charged, thus placing apositive bias on the lead 8 and thereby causing an increased flow of current through the tone circuit of the corresponding plate 5; This increase in current fiow, when it traverses. .the resistor, causes an increased positive voltage to appear .at the cathode end of the resistor I4, which biasis transmitted to the grid l8 of the amplifier tube l6 thereby rendering the amplifier more conductive. The oscillation producedby the tone generator I5 is thus permitted to modulate the plate current of the amplifier tube which causes a corresponding audible note to be provided at the earphones 22.

As the key 3| proceeds downward, it moves the contact spring .40 out. of engagement. with the contact. 42, whereupon the bucket condenser 9 discharges through its resistor I, thus removing the positivev bias fromthe lead 8 of the tone circuit, whereby the note produced in the earphones 22 is attenuated so as to produce a sound similar to the note of,.the..piano. When the contact spring engages ,thecontact 44,;the tank condenser 38 is dischargedand remains in this condition until thekeyis released thereby causing the tank condenser to be recharged. A resistor 41 of small-.-value is preferablyplaced in the ground lead of the tank condenser 38 to prevent thumping when the, key is depressed.

There will be a slight dwell of the contact 42 against the contact spring 40 ;as the key is depressed, and during .thattime interval the resister 46 bleeds off a part of, the charge on the two condenserswhich are at that time connect-ed. If the key is struck sharply, the attenuation of the signal caused .by.v the resistor 46; will be negligible and the note will be of maximum loudness, whereas if the key is depressed slowly the note will be reduced in volume.

It. will be understood that although each tone circuit is only connected to the sound amplifying system for a very short time interval, since the electronic switch operates at a supersonic frequency, the pulses of coupling occur at such a high frequency that the tone envelope appears to be continuous to the listener.

Although but one simplified form of the invention hasbeen shown and described, it will be understood that the basic principles of the invention may be embodied in other forms, and that changes may be made in the specific structural features and arrangements without departing from the spirit of the invention as defined in the claims appended hereto.

What is claimed is:

1. In an electronic keyboard instrument, a plurality of standard frequency tone oscillators, an amplifier and a load device, means normally rendering the amplifier inoperative, an electronic switching device for periodically connecting said oscillators to the amplifier in sequence, aplurality of keys corresponding to said oscillators, and means actuated by each keyfor rendering the amplifier operative at those times when it is connected to the oscillator corresponding to said key; including further, means for attenuating the signal while the key ishcld down, and for extinguishing the signal when the key is released.

2. In an electronic keyboard instrument, a cathode-ray tube having an electron-emitting cathode, means forming the electrons emitted thereby into a beam and projecting them on a target, and a plurality of spacedconductive plates in the target; means causing the beam of electrons to periodically impinge on said plates in sequence at a supersonic frequency, a return circuit fromv each plate to the cathode of the tube including a common resistor, means for impressing an audiofrequency tone oscillation on each plate circuit, a sound amplifying system including a thermionic tube modulated by the signal appearing as voltage drop across said common resistor, means normally biasing said tube to prevent said signals from actuating the sound amplifying system, a key corresponding to each plate return circuit, and means actuated by said keys for placing a positive voltage on the corresponding plate sufficient to cause the signal to actuate the sound amplifying system.

3. An electronic keyboard instrument as set forth in claim 2 in which each of said key-actuated means includes a condenser connected to the corresponding plate, means responsive to actuation of the key for so chargingthe condenser as to place a positive bias on the plate, and then disconnecting the charging, means, and a resistor bridging the condenser for bleeding ofi said charge.

THEODORE K. RIGGEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,250,065 Koehl July 22, 1941 2,250,528 Gray July 29, 1941 2,293,499 Fisher Aug. 18, 1942 

